This invention relates to a system for maintaining desired moisture levels in cut tobacco during the process of drying the tobacco. Particularly, this invention relates to comparing data from a plurality of infrared moisture meters with data from analytical moisture tests for a plurality of blends of tobacco, wherein a statistical analysis is performed on said data to produce a calibration curve for each moisture meter for each blend of tobacco. This calibration curve data is automatically transferred to a control system programmed to control the process of drying, where the raw data coming from each moisture meter is adjusted in accordance with the calibration curve for each blend of tobacco before the meter data is used to control further processing of the tobacco.
In the tobacco industry, the determination of moisture content of cut tobacco is of critical importance. Frequently, the moisture content of tobacco leaf is measured with a reflectance-type infrared absorption meter, which determines the water content of the tobacco by measuring its infrared reflectance in response to two beams of light in the near infrared region. One of the beams of light is within one of several infrared wavelength bands characteristic of water absorption (i.e., 0.8-2.5 microns). The other beam of light is just outside whatever wavelength band that is used for the first beam. The difference in the reflectance values of the two beams is a function of the moisture content of the sample.
Because of drift characteristics commonly seen in metering circuitry, moisture meters must be recalibrated periodically to verify that the low and high readings have not changed from a standard. Calibration may be accomplished by comparing a meter's response to a standard material having known reflectance values, and either adjusting the meter or devising a corrected scale in response thereto. Typically, a meter is adjusted for regulation of a span between the "0" point on the instrument and the upper reach of values to meet technical specifications in a given range of physical measurements. Some commercially supplied standard test materials include simple black-and-white reflectance disks that are intended to provide reproducible low and high reflectance values. However, black-and-white reflectance testing has generally been found unsatisfactory due to uncontrolled specular and diffuse reflections. Instruments may also be calibrated using actual samples of a particulate matter that are believed to cover the range of desired moisture content. U.S. Pat. No. 4,082,950 to Chen teaches a calibration assembly where the reflective sample material is sandwiched between two etched glass plates and is used for comparison to calibrate a meter for the desired range.
Another type of calibration relies upon a statistical comparison of metered and laboratory-tested data to produce a cross-reference table, or "calibration curve," which expresses a linear relation between the meter readings of the subject matter and the readings of a know comparable matter, in order to adjust the meter. Typically, a calibration curve is derived by comparing meter readings from several samples to laboratory test results on the same samples, where each reading measures the same characteristics of the matter so as to construct a cross reference relationship. Mathematically, a calibration curve may be graphed as a smooth curve connecting a series of calibration points that relate the experimentally determined test sample data to the electronically determined meter values. Because of minor variations from one meter to another, a unique calibration curve must be calculated for each meter. In addition, the calibration curve for each meter must be further calibrated with each blend of tobacco.
U.S. Pat. No. 4,864,842 to Regimand teaches a method and system for transferring calibration data to nuclear gauges in the field that are used to measure neutron moderating characteristics of sample materials, such as asphalt-aggregate paving mix. Regimand teaches the cross-referencing of output from field gauges to output from a master gauge adjusted to a known standard to create a calibration curve. This curve data may be manually entered into the memory of a field gauge to avoid the necessity of individually transporting the gauges back to the laboratory for calibration.